Recent optical semiconductor devices have been improved in efficiency, so that the applications thereof have been wide-spread not only to light sources for display devices such as a backlight for a liquid display device but also to the lighting fixture field including general lighting fixtures, lights for vehicle lights, and the like.
For example, if such an optical semiconductor device is applied to a backlight light source for a liquid display of a cell phone, the optical semiconductor device can be driven with a current of approximately 20 mA. On the contrary, if such an optical semiconductor device is applied to a lighting fixture for a vehicle light, for example, the optical semiconductor device can be driven with a current of approximately 1 A. The increase in driving current can cause higher heat generation from the optical semiconductor device. If an optical semiconductor apparatus utilizes such a high output optical semiconductor device, certain measures must be taken for efficient dissipation of generated heat.
A known optical semiconductor device disclosed in Japanese Patent No. 3896044 includes an optical semiconductor layer and a metal plate with high heat conduction formed on the optical semiconductor layer while a sapphire substrate has been removed, thereby improving the heat dissipation property.
FIG. 1 is a flow chart showing a conventional manufacturing process of an optical semiconductor device with a metal plate and an optical semiconductor apparatus including the same.
The manufacturing process includes a semiconductor layer formation step, a metal support formation step, a growth substrate removal step, an n-electrode formation step, and a separation step (chipping step). First, in the semiconductor layer formation step a semiconductor layer is formed on a growth substrate. The semiconductor layer can be formed by, for example, metal organic chemical vapor deposition method (MOCVD). Next, in the metal support formation step, a metal film is deposited on the semiconductor layer, and then, the metal film is plated to form a metal plate (plating film). In the growth substrate removal step, the growth substrate is removed to expose the semiconductor layer. Then, in the n-electrode formation step, an n-electrode is formed on the exposed semiconductor layer, and the semiconductor layer and the metal plate are separated together in the separation step, thereby completing individual optical semiconductor devices as chips.
Furthermore, the optical semiconductor device is packaged to be utilized as an optical semiconductor apparatus by die-bonding, wire-bonding, and resin sealing. In the die-bonding step, the metal plate of the optical semiconductor device is fixed to one of a pair of stems via a conductive adhesive. In the wire-bonding step, the n-electrode is electrically connected with the other stem via a gold (Au) wire. In the resin sealing step, at least part of the stems, the optical semiconductor device and the Au wire are sealed with a light transmitting resin, thereby manufacturing the optical semiconductor apparatus.
The conductive adhesive in the die-bonding step can be an AuSn solder (Au: 80 wt %) with favorable heat conduction and electric conduction, for example. The AuSn solder is applied to the stem or the optical semiconductor device, followed by heating it in a reflow furnace at approximately 315° C. to form an AuSn eutectic junction. During this process, the semiconductor layer may be broken by forces applied to the semiconductor layer due to the difference in thermal expansion coefficient between the semiconductor layer and the metal plate, and the density distribution unevenness of the inside of the metal plate.
If the Au content in the AuSn solder is set to 10 wt %, for example, the eutectic temperature can be lowered to approximately 210° C. In this case, the problem in which the semiconductor layer is broken can be prevented to a certain extent. Even when the difference in thermal expansion coefficient between the semiconductor layer and the metal plate is not changed, if the Au content is decreased, the eutectic junction can be formed within a temperature range in which the thermal expansion of the metal plate cannot break the semiconductor layer.
However, if such an AuSn solder having a low Au content is used for manufacturing an optical semiconductor apparatus, when the manufactured optical semiconductor apparatus is heated (for example, 250° C.) to be soldered to a printed board or the like, the optical semiconductor device or stems may be peeled off. In this case, the device failure may occur. This is because the AuSn solder with the low Au content can be re-melted at the temperature of 250° C., thereby significantly weakening the bonding strength between the optical semiconductor device and the stems.